In recent years, mobile telephones, mobile data devices, and the like have spread rapidly with the development of mobile communication systems. In addition to mobile telephones rapidly developing towards being multiband and multimode, wireless interfaces such as wireless LAN, Bluetooth (trademark), and GPS are increasingly included in mobile telephones, and the miniaturization and integration of the RF circuit in mobile telephones is in strong demand. In this situation, miniaturization and higher performance is increasingly demanded of the duplexer, which is used as an antenna duplexer or the like and is an important component in the RF circuit.
The duplexer, which is used as an antenna duplexer or a branching filter, separates the received signal and transmission signal, which differ in frequency, by relying on the difference in impedance. If there is one type of the transmission signal and one type of the received signal, then the duplexer is provided with one transmitting filter and one receiving filter. There is a possibility of having three or more filters, and in a case in which there are three filters, for example, the device is called a triplexer, but in the present application, all such devices are referred to as a duplexer. A one-port resonator is often used for these filters. Recently, as stated in Patent Document 1, a miniature film bulk acoustic wave resonator (FBAR) or a surface acoustic wave (SAW) resonator is often used instead of the previously widely used dielectric resonators, which are large in size.
The surface acoustic wave resonator is constituted of an interdigital transducer (IDT) that is connected to an input terminal and an output terminal on a piezoelectric substrate, and, if necessary, reflectors disposed on both sides of the IDT. The IDT and the reflectors are made of a metal such as aluminum (Al).